Drivers not updating on "display" field
OK i found the issue.
The Display field only updates when you change frame, it will not update if you move the empty but stay on the same frame, if you move one forward or back it works.

You can also use python to control this field. ( Firstsplash is name of particle settings, replace with whatever the name of you particle settings is )

import bpy

bpy.data.particles["Firstsplash"].draw_percentage = 1
bpy.data.particles["Firstsplash.001"].draw_percentage = 1
bpy.data.particles["Firstsplash.003"].draw_percentage = 1